For writers, it's much more difficult to write (well, at least) something he or she knows very little to nothing about than it is to write something he or she knows very well. If, say, a writer has never owned a cat, the collection of original humorous stories he writes about the funny things cats do will probably fall flat with avid cat lovers. The only way to make it work would be go out and do the leg work to actually get real-life anecdotes from those crazy cat people.
Writing believable characters is essential to telling a good story. Even if your character is a two-headed pig-frog hybrid creature from the planet Ultron, it still needs to be believable in the context and environment it lives in. We've all had those moments watching a television show when we said to ourselves, "That newborn baby is NOT a newborn baby!" or "Who are they kidding!? Those are NOT teenagers!"
When you throw teenage characters into the mix things become even more complicated. Anyone who has spent any time with teens--either their own or someone else's--knows that attitudes, even personalities, seem to change on a daily basis. And that is exactly what is happening, too. The teen years, according to psychologists, is a time when humans subconsciously "try on" different personalities to see the types of reactions they'll receive. Of course, very few teens themselves know this is happening. All they know is that they are different people at home, school, with friends, and with other adults. This makes writing a truly believable teen difficult, but there are ways to help you get the best teen character.
Of course, if you are a teen (or just out of your teenage years), write what you know; write your teen characters as you think/thought and feel/felt. This is the easiest way to write believable teens.
If, however, you're like most of humanity, you are not a teenager. While that's a fairly high hurdle to overcome, there are still ways to get around your adult-type thinking. Simply going to where youth hang out and listening in on conversations will not get you the point of view you're seeking, especially if you're writing form a first person or omniscient point of view and want to include realistic inner dialogue. Even with their friends, few teens open up and share their innermost thoughts. Simply going to where there are a lot of teenagers and listening in will not give you the inner dialogue you want.
The best way to write a teenage character--even if it may take months to even years--is to get to know a teenager enough that he or she trusts you enough to let you in and really talk to you. Because of the teen's inner struggle, opening up and showing "the real me" is difficult for them. They will not easily trust someone if they suspect that trust will be violated. To show yourself worthy of their trust, you must get to know them. You need to be willing to listen to the inane prattle and the surfacey fluff and not pass judgement before they will open up and show you how they really think (within reason, of course--you shouldn't, by your lack of judgment, be seen as encouraging dangerous, destructive, or illegal behavior).
Volunteering in a youth program is one of the best ways you can accomplish this. If you are religious, ask your house of worship's youth director if they need help (as one myself, I can tell you with absolute certainty: they will always need someone to help, especially with middle schoolers). If you are non-religious, there are still plenty of places to go to begin relationships with youth. You could be a Big Brother or Big Sister. Police departments in cities usually have teen outreach programs. Volunteer at a school or the YMCA. If you already have a book published, ask school librarians if they would like you to come in and do a reading. Build relationships with the kids. Take them (with parental permission of course) out to a game, or for a coffee, or just out with you running errands. Any time spent with them one-on-one or one-on-two will be good for not only you, but for them as well.
Speaking from personal experience, once the relationship is built, you will be able to pick their brains, and they can even help you with your story. Young Adult Fiction authors have some of the best resources (and story editors) available in the angsty teenager. Few, however, take the time to mine this resource, and many times, characters simply come out sounding like adults.